Saturday, 17 November 2012

Hard science: the future of crop protection

Posted by
Ben Hall

Food security, whilst something of a buzzword, is one of
the great challenges facing this generation and every generation for the
foreseeable future. A major threat to global security is pestilence. Total
yield lost to pathogens and pests is an immensely difficult thing to quantify.
It’s subject to a whole host of variables but a decent estimate would be
something in the region of 25% globally. Gone are the days when crops could be
liberally doused with chemicals designed to kill the disease causing agent as
standard procedure, largely because these chemicals have an annoying tendency
to kill things we’d really rather keep alive! Not to mention the remarkable
speed at which populations of pathogens resistant to our chemical warfare
emerge. In the future, a smarter approach is required, one which will probably
involve genetic engineering. Those best placed to provide the solutions are
molecular biologists working at the interface between pathogens and their
hosts.

In the contemporary plant pathology scene, the major
paradigm is the study of so-called ‘effectors’. An effector is a small
molecule, often a protein, which is secreted into the plant cell by microorganisms
with the aim of eliciting some kind of response in the host which will benefit
the pathogen. However, the pathogen certainly doesn’t get it all its own way.
Plants, unlike animals, don’t have an army of motile immune cells capable of
rushing to infected tissue to repel invasion. Plant cells, as a rule, don’t
move. Sure, they possess a fully functioning vascular system roughly analogous
to an animal circulatory system, which transports water, sugars, hormones and
other signalling molecules but it most certainly does not transport immune
cells. What they do have is a very efficient surveillance system capable of
recognising signals associated with pathogens and then producing an immune
response.The first layer of resistance recognises microbe-associated
molecular patterns (MAMPs for short), which are patterns which microorganisms
produce on account of being microorganisms. A good example is flagellin, a
protein involved in cell motility. MAMP recognition triggers a weak immune
response. Why bother with a weak immune response though? If you know you’re
under attack surely it’s better to hit the attacker with everything you’ve got?
Well, not necessarily. The fact is that all microbes produce MAMPs but not all
microbes produce disease. As a ‘strong’ plant immune response tends to involve
localised cell death, killing your own cells every time you recognise a microbe
would be a terrible strategy in terms of evolutionary fitness. This is where
the second layer of resistance comes in, a resistance which concerns the
effector proteins I mentioned earlier. One major function of effectors is the
subversion of MAMP triggered immunity in order to facilitate infection. However,
as well as recognising MAMPs, plants are also capable of recognising effector
proteins through the production of recognition proteins from genomic regions
termed ‘resistance (R) genes’. Pathogens produce a diverse array of effectors
and plants possess an equally diverse set of R genes, the system is a classic
example of an evolutionary ‘arms race’, a race which the pathogens most
definitely seem to be winning! Recognition of an effector by the product of an
R gene often leads to a robust immunity in the form of the hypersensitive
response, a localised cell death signal which restricts the growth of pathogens
that require living tissue to complete their life cycles. Unfortunately, many
effector proteins are not recognised by R gene products and are free to exert
their effects on the host. In order to
develop resistance to important crop pathogens we need to fully understand how
effectors produce their effects in the host cell and how effectors and R
proteins interact. Knowledge of these processes remains thin on the ground so
current research is very much focused in their direction.One fruitful avenue of research is the structural
characterisation of the key players involved in this system. It’s parsimonious,
then, that the 1000th crystal structure determined at the Diamond
Light Source, Oxford, happened to be that of an effector protein secreted by
the tomato pathogen Pseudomonas syringae,
which was published in the journal PNAS last month. Scientists from the
John Innes Centre, using the x-ray diffraction facility at Diamond, were able
to figure out the structure of the protein, known as AvrRPS4, by inference from
the electron density pattern it produces when x-rays are directed at the
protein and diffracted. This structural information was then used to inform the
generation of mutant effector proteins which were no longer recognised by the
associated R protein and of mutants which were able to interact with the R
protein whilst still inducing resistance but without any associated cell death.

The Diamond Light Source, Oxfordshire

The implications of this are pretty cool. Firstly, it
tells us which parts of the effector are needed for recognition by the host.
Secondly, and more importantly, it adds to an emerging body of evidence
suggesting that cell death is not a requirement for resistance. This is a big
deal! Resistance without cell death is extremely desirable in terms of food
production and this research represents a massive step toward developing it
and, ultimately, deploying it in real crop production situations.

As an aside, structural study of effector proteins also
allows an incredible snapshot of how protein structure underpins the incredible
rate at which these ‘molecular weapons’ evolve. And, trust me, they evolve
extremely quickly!For those interested who have access to PNAS, the
reference for the full study is as follows: